The present invention relates to a process for the non-destructive inspection of surface defects, in that initially a solution with one or more dyes essentially having no emission under UV-light is applied and excess solution is removed after the dye has penetrated the surface defects and then the surface is covered with a developer leaving behind an absorbent coating and which contains at least one fluorescent component, the surface defects being revealed under UV-light as blackened portions within the light-emitting ambient.
For the non-destructive inspection or detection of surface defects, particularly on metal surfaces, the so-called red/white process is frequently used and consists of applying a solution with a dye having a high contrast in daylight to the surface, so that solution and dye can penetrate the surface defects. The excess solution is then removed from the surface, so that no dye residues are left behind. A coating is then applied, which leaves behind a background contrasting with the dye. For example, it can be a solvent with dispersed talc. After drying the coating, and surface defects are revealed by the coloring of the absorbent talc with the dye left behind in the surface defects.
A supplement to this process has been proposed for showing very small surface defects, such as cracks and the like (U.S. Pat. No. 3,564,249). In this process, following the wiping off of the dye applied, a developer is applied, which once again leaves behind a contrasting, e.g. approximately white and absorbent coating. This developer additionally contains a fluorescent component, which emits under UV-light. The absorbent component of the coating, which can also consist of talc, once again removes the dye from the surface defects. A complementary color is formed at the corresponding points from the dye and the fluorescent component and appears as a blackened portion under UV-light. These blackened portions can be detected clearly and in a sharply defined manner even when the red/white process leaves behind no or only unclear colorings. It is also pointed out that it need not be exclusively a red dye and also dyes in the blue range can be considered.
The known developer should contain approximately 1.5% of the essential component Fluoral 7 GA (product of General Aniline and Film Corporation). It is also proposed to add 3.2% of an optical brightener, in addition to the fluorescent dye.
Fluoral 7 GA and similar fluorescent dyes give the coating a striking yellow coloring, which impairs contrast in daylight. The problem to be solved by the present invention is to permit a better contrast within the coating.